Syphilis is the third ranking reportable communicable disease in the United States; litle is known, however, about the host's immune response to the infecting organism, Treponema pallidum. The experimental infection of he rabbit with this organism is the laboratory has been investigating and characterising the rabbit's immune system for over ten years and will apply this expertise to a detailed study of the T. pallidum infection in the rabbit. A pilot study will be performed which will include serologic tests for syphilis immunofluorescent microscopy of numerous organs to determine the T. pallidum, T cell and Becell distribution in those tissues, blast transformation of whole and functionated lymphocyte populations b known mitogens and T. pallidum preparations, protective tests by passive local immunization, and the measurement of anti-T. pallidum antibody-forming cells as well as cell which exhibit an in vitro treponemicidal activity. Following the evaluation of these parameters, similar studies will be undertaken on immunologically compromised aimals such as neonates, thymectomized adults and cortisn-treated animals. In addition, the late lesions of experimental syphilis will be examined by following the effects of infection over a 3-4 year period. It is anticipated that these studies will further the understanding of the hosttreponeme interactins in syphilis and provide insights which can eventually be applied toward developing an effective and practical vaccine.